Schematic
This section contains the detailed electronic schematic of the Archean synthesizer. The schematic diagram is an essential resource for understanding how the circuit components are connected and how the overall system operates.
The schematic is available as a downloadable PDF file on GitHub. It represents the entire circuitry including:
This section contains the detailed electronic schematic of the Archean synthesizer. The schematic diagram is an essential resource for understanding how the circuit components are connected and how the overall system operates.
The schematic is available as a downloadable PDF file on GitHub. It represents the entire circuitry including:
- Power supply and voltage regulation
- Oscillator output and signal path
- Filter shaping the sound spectrum
- VCA, overdrive and delay
- Control interfaces such as knobs, sensors, and buttons
- MIDI input
- Reading the schematic may seem complex at first, but it follows standard electronic symbols and conventions. By following the signal flow and component connections, you can gain insight into the design and functionality of the synthesizer.
- We encourage users to reference the schematic if they want to repair, modify, or customize the hardware. Understanding the schematic alongside the source code will provide a comprehensive view of the Archean synthesizer's architecture.
- Please follow common safety precautions when working with electronic circuits. If you are new to electronics, consider studying basic schematic reading skills and circuit theory to make the most out of this resource.
Teensy 4.0
What is Teensy? Teensy 4.0 is a small, powerful microcontroller development board created by
PJRC (Paul Stoffregen) — a tiny computer optimized for real-time electronics projects.
Technical Specifications. Processor: ARM Cortex-M7 (NXP iMXRT1062), 600 MHz, 32-bit. Memory: 2 MB flash, 1024 KB RAM.
Why Teensy 4.0 for Archean?
1. Processing power. 600 MHz is fast enough for audio-rate oscillator interrupts, real-time
control, DMA operations and many simultaneous tasks.
2. Audio performance. Precise timers (IntervalTimer with microsecond precision), a fast ADC with DMA support, and fast SPI (20 MHz) for the three DACs.
3. Communication buses. SPI: one bus shared by three DACs — oscillator; ADSR & distance; LFO & Element. I2C: Wire1 — touch sensors (MPR121 × 2) and distance sensor (VL53L0X). Serial: hardware UART for MIDI input. USB: USB MIDI and programming.
4. Arduino compatibility. Familiar programming environment, huge library ecosystem, cross-platform.
5. Small form factor. 36 mm × 18 mm.
Teensy connections (sheet 1): pin 0 — MIDI in (RX1); 1 — CS of the oscillator DAC; 2 — distance sensor interrupt; 3 — LFO switch; 4 — MPR121 interrupt; 5, 6 — oscillator switch; 7 — button; 8 — LED; 9 — Gate; 10 — CS of the ADSR & distance DAC; 11 — MOSI; 12 — CS of the LFO & Element DAC; 13 — SCK; 14 — Pitch; 15 — Fine; 16, 17 — I2C (SCL1, SDA1); 18 — LFO; 19 — Element; 20 — Release; 21 — Decay; 22 — Sustain; 23 — Attack. VIN — +5V.
PJRC (Paul Stoffregen) — a tiny computer optimized for real-time electronics projects.
Technical Specifications. Processor: ARM Cortex-M7 (NXP iMXRT1062), 600 MHz, 32-bit. Memory: 2 MB flash, 1024 KB RAM.
Why Teensy 4.0 for Archean?
1. Processing power. 600 MHz is fast enough for audio-rate oscillator interrupts, real-time
control, DMA operations and many simultaneous tasks.
2. Audio performance. Precise timers (IntervalTimer with microsecond precision), a fast ADC with DMA support, and fast SPI (20 MHz) for the three DACs.
3. Communication buses. SPI: one bus shared by three DACs — oscillator; ADSR & distance; LFO & Element. I2C: Wire1 — touch sensors (MPR121 × 2) and distance sensor (VL53L0X). Serial: hardware UART for MIDI input. USB: USB MIDI and programming.
4. Arduino compatibility. Familiar programming environment, huge library ecosystem, cross-platform.
5. Small form factor. 36 mm × 18 mm.
Teensy connections (sheet 1): pin 0 — MIDI in (RX1); 1 — CS of the oscillator DAC; 2 — distance sensor interrupt; 3 — LFO switch; 4 — MPR121 interrupt; 5, 6 — oscillator switch; 7 — button; 8 — LED; 9 — Gate; 10 — CS of the ADSR & distance DAC; 11 — MOSI; 12 — CS of the LFO & Element DAC; 13 — SCK; 14 — Pitch; 15 — Fine; 16, 17 — I2C (SCL1, SDA1); 18 — LFO; 19 — Element; 20 — Release; 21 — Decay; 22 — Sustain; 23 — Attack. VIN — +5V.
MIDI In
This circuit converts MIDI signals from the 3.5 mm TRS jack (J2) to 3.3V logic levels for the Teensy 4.0.
Components:
1. The MIDI device sends data: current flows through the cable into the jack.
2. Current flows through R1 into the LED of IC3 (pins 2–3); the LED lights up.
3. The phototransistor detects the light and conducts, pulling the output (pin 6) LOW.
4. Teensy pin 0 (RX1) reads LOW; the UART decodes the MIDI byte.
5. No signal: no current, the LED is off, the pull-up resistor keeps the output HIGH (idle state). Result: a safe MIDI input that works with any MIDI device (with a TRS MIDI cable or adapter).
Result: a safe MIDI input that works with any MIDI device (with a TRS MIDI cable or adapter).
Components:
- R1 — current limiting resistor (220Ω). Limits current through the optocoupler LED.
- D2 — protection diode across the optocoupler LED. Protects the LED from reverse voltage (for example, a wrongly wired cable); no effect in normal operation.
- IC3 — optocoupler (6N138): two separate circuits connected only by light. It gives electrical isolation (prevents ground loops, protects from voltage spikes, stops noise injection), converts levels (MIDI 5V current loop → Teensy 3.3V logic) and protects the Teensy from faulty MIDI devices. Isolation is a MIDI specification requirement.
- R2 — pull-up resistor (220Ω) to +3.3V. Pulls the output HIGH when the LED is off; the phototransistor only pulls LOW.
- R3 — resistor (4.7kΩ) on the base of the output transistor; speeds up switching.
1. The MIDI device sends data: current flows through the cable into the jack.
2. Current flows through R1 into the LED of IC3 (pins 2–3); the LED lights up.
3. The phototransistor detects the light and conducts, pulling the output (pin 6) LOW.
4. Teensy pin 0 (RX1) reads LOW; the UART decodes the MIDI byte.
5. No signal: no current, the LED is off, the pull-up resistor keeps the output HIGH (idle state). Result: a safe MIDI input that works with any MIDI device (with a TRS MIDI cable or adapter).
Result: a safe MIDI input that works with any MIDI device (with a TRS MIDI cable or adapter).
Power Supply
Multi-rail power supply providing +12V, −12V, +5V and +3.3V for different circuit sections. Input: 15V DC external supply.
Input Stage: J1 — power jack (pin 1: +15V, pin 3: ground). D1 — reverse polarity protection. FUSE — overcurrent protection; protects the circuit from shorts. C1 (47μF), C2 (10μF) — input filtering.
Voltage Rails:
Result: stable, quiet power that keeps the Archean running reliably.
Input Stage: J1 — power jack (pin 1: +15V, pin 3: ground). D1 — reverse polarity protection. FUSE — overcurrent protection; protects the circuit from shorts. C1 (47μF), C2 (10μF) — input filtering.
Voltage Rails:
- +15V — directly from the input (after the diode drop, about 14.3V). Feeds the +12V regulator
- and the −12V converter.
- +12V — regulated by IC1 (7812). C3 (0.47μF) — output filter. Supplies: op-amps (TL072, TL074), LM13700, analog circuits.
- +5V — regulated by IC2 (7805) from +12V. C4 (0.1μF) — output decoupling. Supplies: Teensy 4.0 (VIN), PT2399, MCP6004 input op-amps.
- −12V — DC-DC converter module U1, from +15V. C5 (0.47μF) — output filtering. Supplies: op-amps, LM13700, the −10V reference.
- +3.3V — from the regulator on the Teensy. Supplies: DACs, MPR121 touch sensors, knobs, the 2.5V reference.
Result: stable, quiet power that keeps the Archean running reliably.
Voltage References
These circuits generate stable, precise reference voltages used throughout the Archean for accurate CV processing. Three voltage references:
- VREF2.5 — 2.5V reference: U3 (LM4040) with R44 (1kΩ) from +3.3V. Reference for all three DACs.
- AREF-10V — −10V reference: U2 (LM4040) with R43 (330Ω) from −12V, C9 (0.1μF). Feeds the Tune, Element and LFO knobs.
- V_BIAS — 1.0V bias: divider R41 (12kΩ) / R42 (8kΩ) from VREF2.5, C10 (0.1μF). Offset for the bipolar CV outputs.
CV Inputs
This circuit is a signal conditioning stage for the Control Voltage (CV) inputs that safely interfaces them with the ADC pins of the Teensy 4.0.
Why is this circuit needed? Modular synthesizer CV signals are commonly bipolar (−5V to +5V) or up to several volts positive (1V/Oct). The Teensy 4.0 ADC pins accept only 0V to 3.3V and can be damaged by voltages outside this range. This circuit scales the input signal, adds the knob position as an offset, filters it and protects the ADC pin. There are three channels: Pitch (J4, Tune knob VR1), Element (J5, VR2) and LFO (J6, VR3). All op-amps are MCP6004 (IC4, IC5), powered from +5V.
Signal flow and stage analysis (Pitch channel):
Element and LFO channels are built the same way (IC5B → IC4B with VR2; IC5C → IC4C with VR3): the first stage has a gain of −27k / 100k = −0.27, the second −33k / 27k = −1.22, so the input is scaled by 0.33; the knob adds 0 to +3.3V through 100kΩ (R15, R21); 470pF capacitors (C8, C7) filter at about 10 kHz; R17, R23 (10Ω) and Zener diodes D8, D9 protect the ADC. A ±5V input fits the ADC range with the knob in the middle.
Why is this circuit needed? Modular synthesizer CV signals are commonly bipolar (−5V to +5V) or up to several volts positive (1V/Oct). The Teensy 4.0 ADC pins accept only 0V to 3.3V and can be damaged by voltages outside this range. This circuit scales the input signal, adds the knob position as an offset, filters it and protects the ADC pin. There are three channels: Pitch (J4, Tune knob VR1), Element (J5, VR2) and LFO (J6, VR3). All op-amps are MCP6004 (IC4, IC5), powered from +5V.
Signal flow and stage analysis (Pitch channel):
- Input (J4 jack). The CV signal enters through the tip and R6 (100kΩ).
- First op-amp stage (IC5D) — inverting attenuator. Gain = −R7 / R6 = −24k / 100k = −0.24.
- Second op-amp stage (IC4A) — inverting summing amplifier. It adds two signals: the first stage through R8 (12kΩ, gain −R9 / R8 = −2) and the Tune knob VR1 through R10 (160kΩ). VR1 sits between AREF-10V (−10V) and ground, so it adds an offset of 0 to +1.5V (−R9 / R10 × 0...−10V).
- Filtering. C6 (470pF) across R9 (24kΩ) forms a low-pass filter at about 14 kHz.
- Protection. R11 (10Ω) and the 3.3V Zener diode D7 (1N4728) limit the voltage at the ADC pin.
Element and LFO channels are built the same way (IC5B → IC4B with VR2; IC5C → IC4C with VR3): the first stage has a gain of −27k / 100k = −0.27, the second −33k / 27k = −1.22, so the input is scaled by 0.33; the knob adds 0 to +3.3V through 100kΩ (R15, R21); 470pF capacitors (C8, C7) filter at about 10 kHz; R17, R23 (10Ω) and Zener diodes D8, D9 protect the ADC. A ±5V input fits the ADC range with the knob in the middle.
DAC for Oscillator
This circuit generates the main audio signal, converting Teensy's digital waveform data into an analog voltage.
Stage 1: SPI Communication from Teensy. CS (pin 1), SCK (pin 13), MOSI (pin 11). The Teensy pulls CS low, sends a 16-bit word via MOSI with clock pulses on SCK, then pulls CS high; the DAC latches the data and updates its output.
Stage 2: DAC Chip (IC11 — MCP4921). 12-bit voltage output DAC: 4096 steps, SPI interface (20 MHz capable), single channel, external reference input, rail-to-rail output, low glitch energy. Powered from +3.3V; LDAC tied to ground, so the output updates when CS goes high.
Stage 3: Voltage Reference (VREF2.5). The DAC output range is set by its reference: 0–2.5V. Stability = pitch stability; low noise = clean audio.
Stage 4: Reconstruction Filter (IC12A — TL072). R50, R51 (18kΩ), C26 (470pF, to ground) and C27 (470pF, feedback) with IC12A as a unity-gain buffer form a second-order (Sallen-Key) low-pass filter: fc ≈ 1 / (2π × 18kΩ × 470pF) ≈ 18.8 kHz. It removes DAC steps and switching artifacts, filters SPI clock feedthrough and still passes the audio range. The buffer drives the next stage so the DAC provides minimal current.
C28 — AC coupling capacitor (10μF). Blocks the DC component of the DAC output (0–2.5V) so the audio is centered at 0V. With R52 it passes frequencies down to below 1 Hz.
Stage 5: Amplifier (IC12B). Inverting amplifier. R52 — input resistor (24.9kΩ). R53 — feedback resistor (100kΩ). Gain = −R53 / R52 = −100k / 24.9k ≈ −4. The 2.5V peak-to-peak DAC signal becomes about 10V peak-to-peak (±5V) — modular synth level and a better signal-to-noise ratio. C29 (100pF) in parallel with R53: fc = 1 / (2π × 100kΩ × 100pF) ≈ 15.9 kHz — prevents high-frequency oscillation and reduces noise. The output (VCO_OUT_FILTER_IN) goes to the filter.
Stage 1: SPI Communication from Teensy. CS (pin 1), SCK (pin 13), MOSI (pin 11). The Teensy pulls CS low, sends a 16-bit word via MOSI with clock pulses on SCK, then pulls CS high; the DAC latches the data and updates its output.
Stage 2: DAC Chip (IC11 — MCP4921). 12-bit voltage output DAC: 4096 steps, SPI interface (20 MHz capable), single channel, external reference input, rail-to-rail output, low glitch energy. Powered from +3.3V; LDAC tied to ground, so the output updates when CS goes high.
Stage 3: Voltage Reference (VREF2.5). The DAC output range is set by its reference: 0–2.5V. Stability = pitch stability; low noise = clean audio.
Stage 4: Reconstruction Filter (IC12A — TL072). R50, R51 (18kΩ), C26 (470pF, to ground) and C27 (470pF, feedback) with IC12A as a unity-gain buffer form a second-order (Sallen-Key) low-pass filter: fc ≈ 1 / (2π × 18kΩ × 470pF) ≈ 18.8 kHz. It removes DAC steps and switching artifacts, filters SPI clock feedthrough and still passes the audio range. The buffer drives the next stage so the DAC provides minimal current.
C28 — AC coupling capacitor (10μF). Blocks the DC component of the DAC output (0–2.5V) so the audio is centered at 0V. With R52 it passes frequencies down to below 1 Hz.
Stage 5: Amplifier (IC12B). Inverting amplifier. R52 — input resistor (24.9kΩ). R53 — feedback resistor (100kΩ). Gain = −R53 / R52 = −100k / 24.9k ≈ −4. The 2.5V peak-to-peak DAC signal becomes about 10V peak-to-peak (±5V) — modular synth level and a better signal-to-noise ratio. C29 (100pF) in parallel with R53: fc = 1 / (2π × 100kΩ × 100pF) ≈ 15.9 kHz — prevents high-frequency oscillation and reduces noise. The output (VCO_OUT_FILTER_IN) goes to the filter.
CV Output DACs
Dual-channel DACs generating the control voltage outputs. Two identical MCP4922 chips (labelled MCP9422 in the schematic): IC8 — Element CV and LFO CV; IC6 — ADSR and Proximity (distance) CV.
Stage 1: MCP4922 Dual DAC (IC8). 12-bit, two independent channels (DACA and DACB), SPI interface (20 MHz capable), external reference inputs REFA/REFB connected to VREF2.5 (2.5V), supply +3.3V. Chip select: CS_VULCANO (Teensy pin 12).
Element CV Output (channel B, IC9B — TL072P). Configuration: inverting amplifier with offset.
IC6 (chip select CS_ADSR, Teensy pin 10) is built the same way: channel A → IC7B → PROXIMITY_CV_OUT (J7), about ±5V; channel B → IC7A → ADSR, to the VCA. IC7A has a 2.0V offset from the divider R28 (3.9kΩ) / R29 (16kΩ) instead of V_BIAS, so the envelope ranges from about 0V to +10V.
How It Works: the Teensy sends data via SPI; the MCP4922 updates DACA or DACB (0 to 2.5V); the op-amp inverts, amplifies and filters it; a clean CV appears at the output jack.
Result: CV outputs for expanding the Archean with external modular gear — endless patching possibilities.
Stage 1: MCP4922 Dual DAC (IC8). 12-bit, two independent channels (DACA and DACB), SPI interface (20 MHz capable), external reference inputs REFA/REFB connected to VREF2.5 (2.5V), supply +3.3V. Chip select: CS_VULCANO (Teensy pin 12).
Element CV Output (channel B, IC9B — TL072P). Configuration: inverting amplifier with offset.
- R34 (24.9kΩ) — input resistor from DACB to the inverting input (pin 6).
- Non-inverting input (pin 5) — V_BIAS (1.0V), sets the output offset.
- R35 (100kΩ) — feedback resistor; with R34 sets the gain: −100k / 24.9k ≈ −4.
- C21 (470pF) — in parallel with R35: low-pass filter, fc = 1 / (2π × 100kΩ × 470pF) ≈ 3.4 kHz. Smooths CV changes and removes high-frequency noise.
- R36 (220Ω) — output resistor: short-circuit protection.
- J9 — ELEMENT_CV_OUT, 3.5mm output jack.
- Output = V_BIAS × 5 − 4 × V_DAC: a DAC value of 0 gives about +5V, 2048 gives 0V, 4095 gives about −5V.
IC6 (chip select CS_ADSR, Teensy pin 10) is built the same way: channel A → IC7B → PROXIMITY_CV_OUT (J7), about ±5V; channel B → IC7A → ADSR, to the VCA. IC7A has a 2.0V offset from the divider R28 (3.9kΩ) / R29 (16kΩ) instead of V_BIAS, so the envelope ranges from about 0V to +10V.
How It Works: the Teensy sends data via SPI; the MCP4922 updates DACA or DACB (0 to 2.5V); the op-amp inverts, amplifies and filters it; a clean CV appears at the output jack.
Result: CV outputs for expanding the Archean with external modular gear — endless patching possibilities.
Filter
Voltage Controlled Filter based on the Korg MS-20 design — one of the most iconic synthesizer filters ever made. Purpose: aggressive, resonant filtering with character and attitude.
What is the MS-20 Filter? Original design from the 1978 Korg MS-20 synthesizer, famous for aggressive, screaming resonance. The MS-20 has high-pass and low-pass filters; Archean uses the low-pass. Character: raw, aggressive, in-your-face analog sound.
Configuration: 2-pole (12 dB/octave) low-pass built on two transconductance amplifiers (LM13700N, IC13A and IC13B). A transconductance amplifier (OTA) gives an output current proportional to its input voltage; its gain is set by a control current — perfect for voltage-controlled filters.
Stage 1: First pole (IC13A, C30, IC14A). The audio (VCO_OUT_FILTER_IN) enters through the divider R54 (10kΩ) / R55 (220Ω), which reduces it to a level the OTA can handle. R56 (220Ω) — at the other OTA input. The OTA charges C30 (1nF); IC14A (TL074P) buffers the voltage and feeds it back to the input through R58 (10kΩ).
Stage 2: Second pole (IC13B, C32, IC14B). The same structure: R59 (10kΩ) / R60 (220Ω) at the input, R61 (220Ω), C32 (1nF), buffer IC14B, feedback R63 (10kΩ). The output goes through C33 (0.47μF) to the VCA (FILTER_OUT_VCA_INPUT).
Stage 3: Resonance (IC14C, LED2, LED3, VR11). The filter output goes through VR11 (Q) to IC14C. Its gain (1 + R65 / R64 = 1 + 10kΩ / 1.8kΩ ≈ 6.6) is limited by two LEDs in anti-parallel (LED2, LED3) in the feedback loop — a soft limiter that gives the resonance its MS-20 character. The output of IC14C drives the bottom of C30, closing the resonance loop. More feedback = more resonance = a peak at the cutoff frequency.
Stage 4: Cutoff control (Q5, Q6, IC14D). The cutoff knob VR10 (between +12V and −12V) through R67 (100kΩ) and the CV input J17 through R66 (100kΩ) are summed at the base of Q5 (R68, 1.8kΩ, to ground). Q5 and Q6 with IC14D (R70 4.7kΩ, C31 4.7nF) form an exponential converter that turns the control voltage into the OTA bias current through R57 and R62 (10kΩ). Exponential response: equal steps of voltage give equal musical steps of cutoff.
User Controls: VR10 — cutoff frequency: adjusts brightness from bassy to bright, the most expressive control. VR11 — resonance: emphasis at the cutoff frequency; low — smooth rolloff, high — sharp peak. J17 — CV input: control the cutoff with envelopes, LFOs, sequencers.
MS-20 Filter Characteristics: aggressive resonance — can scream and howl; raw character — slightly dirty, alive; musical distortion — the resonance is softly limited. NOT a smooth Moog-style filter — this is wild and aggressive.
What is the MS-20 Filter? Original design from the 1978 Korg MS-20 synthesizer, famous for aggressive, screaming resonance. The MS-20 has high-pass and low-pass filters; Archean uses the low-pass. Character: raw, aggressive, in-your-face analog sound.
Configuration: 2-pole (12 dB/octave) low-pass built on two transconductance amplifiers (LM13700N, IC13A and IC13B). A transconductance amplifier (OTA) gives an output current proportional to its input voltage; its gain is set by a control current — perfect for voltage-controlled filters.
Stage 1: First pole (IC13A, C30, IC14A). The audio (VCO_OUT_FILTER_IN) enters through the divider R54 (10kΩ) / R55 (220Ω), which reduces it to a level the OTA can handle. R56 (220Ω) — at the other OTA input. The OTA charges C30 (1nF); IC14A (TL074P) buffers the voltage and feeds it back to the input through R58 (10kΩ).
Stage 2: Second pole (IC13B, C32, IC14B). The same structure: R59 (10kΩ) / R60 (220Ω) at the input, R61 (220Ω), C32 (1nF), buffer IC14B, feedback R63 (10kΩ). The output goes through C33 (0.47μF) to the VCA (FILTER_OUT_VCA_INPUT).
Stage 3: Resonance (IC14C, LED2, LED3, VR11). The filter output goes through VR11 (Q) to IC14C. Its gain (1 + R65 / R64 = 1 + 10kΩ / 1.8kΩ ≈ 6.6) is limited by two LEDs in anti-parallel (LED2, LED3) in the feedback loop — a soft limiter that gives the resonance its MS-20 character. The output of IC14C drives the bottom of C30, closing the resonance loop. More feedback = more resonance = a peak at the cutoff frequency.
Stage 4: Cutoff control (Q5, Q6, IC14D). The cutoff knob VR10 (between +12V and −12V) through R67 (100kΩ) and the CV input J17 through R66 (100kΩ) are summed at the base of Q5 (R68, 1.8kΩ, to ground). Q5 and Q6 with IC14D (R70 4.7kΩ, C31 4.7nF) form an exponential converter that turns the control voltage into the OTA bias current through R57 and R62 (10kΩ). Exponential response: equal steps of voltage give equal musical steps of cutoff.
User Controls: VR10 — cutoff frequency: adjusts brightness from bassy to bright, the most expressive control. VR11 — resonance: emphasis at the cutoff frequency; low — smooth rolloff, high — sharp peak. J17 — CV input: control the cutoff with envelopes, LFOs, sequencers.
MS-20 Filter Characteristics: aggressive resonance — can scream and howl; raw character — slightly dirty, alive; musical distortion — the resonance is softly limited. NOT a smooth Moog-style filter — this is wild and aggressive.
VCA
Voltage Controlled Amplifier — controls the audio level using the ADSR envelope, the VCA knob and an external CV. Purpose: shapes volume over time (envelope) and allows expressive control.
Stage 1: Control Voltage Mixer (IC15A — TL072P). Inputs: ADSR (envelope voltage from the DAC), J18 (external CV input), VR12 (VCA knob, manual level from +12V). R73, R74, R75 (100kΩ) — mix resistors. R76 (10kΩ) — feedback resistor. Output CV = −(ADSR + External CV + Manual) / 10. The mixer inverts: a more positive sum gives a more negative output and more current through the VCA core.
Stage 2: VCA Core (Q7, Q8). Configuration: differential pair (long-tailed pair). The output of IC15A sets the current through the pair via R77 (10kΩ). More current → more gain → louder signal; no current → silence. R71 (100kΩ) / R72 (100Ω) reduce the audio 1000 times so the pair works in its linear range. R78, R79 (100kΩ) — collector loads from +12V.
Stage 3: Output Amplifier (IC15B — TL072P). Configuration: differential amplifier. R80, R81 (10kΩ) — input resistors from the two collectors. R83 (470kΩ) — feedback resistor; R82 (470kΩ) — to ground. Gain = R83 / R80 = 470k / 10k = 47× (about 33 dB). R84 (1kΩ) — output resistor. The output (VCA_OUT_DIST_IN) goes to the overdrive.
Control Sources: ADSR envelope (time-varying level), external CV (J18), manual control (VR12).
Result: expressive volume control that brings the synthesizer to life with dynamic amplitude changes.
Stage 1: Control Voltage Mixer (IC15A — TL072P). Inputs: ADSR (envelope voltage from the DAC), J18 (external CV input), VR12 (VCA knob, manual level from +12V). R73, R74, R75 (100kΩ) — mix resistors. R76 (10kΩ) — feedback resistor. Output CV = −(ADSR + External CV + Manual) / 10. The mixer inverts: a more positive sum gives a more negative output and more current through the VCA core.
Stage 2: VCA Core (Q7, Q8). Configuration: differential pair (long-tailed pair). The output of IC15A sets the current through the pair via R77 (10kΩ). More current → more gain → louder signal; no current → silence. R71 (100kΩ) / R72 (100Ω) reduce the audio 1000 times so the pair works in its linear range. R78, R79 (100kΩ) — collector loads from +12V.
Stage 3: Output Amplifier (IC15B — TL072P). Configuration: differential amplifier. R80, R81 (10kΩ) — input resistors from the two collectors. R83 (470kΩ) — feedback resistor; R82 (470kΩ) — to ground. Gain = R83 / R80 = 470k / 10k = 47× (about 33 dB). R84 (1kΩ) — output resistor. The output (VCA_OUT_DIST_IN) goes to the overdrive.
Control Sources: ADSR envelope (time-varying level), external CV (J18), manual control (VR12).
Result: expressive volume control that brings the synthesizer to life with dynamic amplitude changes.
Distortion
Overdrive stage (Weathering) — adds harmonic saturation using diode soft clipping. Purpose: transforms clean audio into harmonically rich, overdriven tones.
Stage 1: Input Coupling (C34). C34 (10μF) — input coupling capacitor: blocks DC from the previous stage and passes only the audio signal.
Stage 2: Op-Amp Gain Stage (IC16A — TL072P). Configuration: non-inverting amplifier; the signal enters the non-inverting input (pin 3). Feedback network: VR13 (Weathering knob), C35 (470pF), diodes D3, D4. To ground: C36 (47nF) in series with R85 (4.7kΩ). Gain = 1 + VR13 / R85 above about 720 Hz (1 / (2π × 4.7kΩ × 47nF)); below this frequency C36 blocks and the gain stays close to 1, so the low end stays clean. C35 reduces the gain at high frequencies and controls fizz.
Stage 3: Diode Clipping (D3, D4 — 1N4148). Configuration: symmetrical soft clipping in the feedback path. When the amplified signal exceeds the diode forward voltage (about 0.6–0.7V), the diodes start to conduct and limit it. Gentle rounding of waveform peaks; symmetrical clipping adds mainly odd harmonics; louder signals clip more.
Stage 4: Output Coupling (C37). C37 (10μF) — output coupling capacitor: removes DC offset and couples the signal to the next stage (delay).
Diode characteristics. Silicon diodes have a lower forward voltage than LEDs, so they clip earlier and more softly. Power supply: the TL072P is powered by ±12V — the signal swings above and below ground.
Result: transforms clean synth tones into harmonically rich, expressive distorted sounds — perfect for leads, basses, and textural elements.
Stage 1: Input Coupling (C34). C34 (10μF) — input coupling capacitor: blocks DC from the previous stage and passes only the audio signal.
Stage 2: Op-Amp Gain Stage (IC16A — TL072P). Configuration: non-inverting amplifier; the signal enters the non-inverting input (pin 3). Feedback network: VR13 (Weathering knob), C35 (470pF), diodes D3, D4. To ground: C36 (47nF) in series with R85 (4.7kΩ). Gain = 1 + VR13 / R85 above about 720 Hz (1 / (2π × 4.7kΩ × 47nF)); below this frequency C36 blocks and the gain stays close to 1, so the low end stays clean. C35 reduces the gain at high frequencies and controls fizz.
Stage 3: Diode Clipping (D3, D4 — 1N4148). Configuration: symmetrical soft clipping in the feedback path. When the amplified signal exceeds the diode forward voltage (about 0.6–0.7V), the diodes start to conduct and limit it. Gentle rounding of waveform peaks; symmetrical clipping adds mainly odd harmonics; louder signals clip more.
Stage 4: Output Coupling (C37). C37 (10μF) — output coupling capacitor: removes DC offset and couples the signal to the next stage (delay).
Diode characteristics. Silicon diodes have a lower forward voltage than LEDs, so they clip earlier and more softly. Power supply: the TL072P is powered by ±12V — the signal swings above and below ground.
Result: transforms clean synth tones into harmonically rich, expressive distorted sounds — perfect for leads, basses, and textural elements.
Delay
Voltage-controlled digital delay based on the PT2399 echo processor (IC17). (In the schematic PDF this sheet is titled Reverb.) Purpose: adds space, echoes and atmospheric depth to the output.
Stage 1: Input. The signal from the overdrive (DIST_OUT_REVERB_IN) enters IC18D (TL074P) through R86 (200kΩ); with R87 (100kΩ) in the feedback the level is halved. C43 (4.7μF) blocks DC; R89 (15kΩ) and C44 (3.9nF) remove high frequencies before the delay.
Stage 2: PT2399. A single-chip digital echo: it converts the audio to digital form, delays it in internal memory and converts it back. Powered from +5V (C38, C39 — decoupling). The filter networks around its internal op-amps (C45–C48, R90–R96) remove clock noise and soften the repeats.
Stage 3: Delay time control (IC18A, Q9). The delay time of the PT2399 is set by the resistance on its VCO pin (pin 6). The Delay knob VR17 (through R99, 120kΩ) and the Delay CV input J19 (through R98, 510kΩ) are summed with an offset from −12V (R100, 270kΩ) in IC18A (feedback R101, 27kΩ). Its output drives transistor Q9 through R102 (470Ω); Q9 changes the resistance on pin 6 (with R103, 1kΩ, and R104, 33kΩ). The delay time is voltage-controlled.
Stage 4: Feedback, mix and outputs. VR14 and VR15 set the feedback (FB) and the dry/wet mix (D/W). VR16 (Volume) feeds the outputs: IC18B and IC18C (TL074P, inverting, gain −1: R105/R106, R108/R109 = 100kΩ) drive the tip and ring of the line output J20 through R107, R110 (100Ω) and C53, C54 (10μF) — the same signal on both, so mono and stereo cables work. IC16B (TL072P, inverting, gain −2: R112 200kΩ / R111 100kΩ) drives the Eurorack output J21 through R113 (1kΩ).
Controls: Delay time (VR17), Delay CV input (J19), FB and D/W (VR14, VR15), Volume (VR16).
Result: echoes from short slapback to long, decaying repeats — with the delay time under
voltage control.
Stage 1: Input. The signal from the overdrive (DIST_OUT_REVERB_IN) enters IC18D (TL074P) through R86 (200kΩ); with R87 (100kΩ) in the feedback the level is halved. C43 (4.7μF) blocks DC; R89 (15kΩ) and C44 (3.9nF) remove high frequencies before the delay.
Stage 2: PT2399. A single-chip digital echo: it converts the audio to digital form, delays it in internal memory and converts it back. Powered from +5V (C38, C39 — decoupling). The filter networks around its internal op-amps (C45–C48, R90–R96) remove clock noise and soften the repeats.
Stage 3: Delay time control (IC18A, Q9). The delay time of the PT2399 is set by the resistance on its VCO pin (pin 6). The Delay knob VR17 (through R99, 120kΩ) and the Delay CV input J19 (through R98, 510kΩ) are summed with an offset from −12V (R100, 270kΩ) in IC18A (feedback R101, 27kΩ). Its output drives transistor Q9 through R102 (470Ω); Q9 changes the resistance on pin 6 (with R103, 1kΩ, and R104, 33kΩ). The delay time is voltage-controlled.
Stage 4: Feedback, mix and outputs. VR14 and VR15 set the feedback (FB) and the dry/wet mix (D/W). VR16 (Volume) feeds the outputs: IC18B and IC18C (TL074P, inverting, gain −1: R105/R106, R108/R109 = 100kΩ) drive the tip and ring of the line output J20 through R107, R110 (100Ω) and C53, C54 (10μF) — the same signal on both, so mono and stereo cables work. IC16B (TL072P, inverting, gain −2: R112 200kΩ / R111 100kΩ) drives the Eurorack output J21 through R113 (1kΩ).
Controls: Delay time (VR17), Delay CV input (J19), FB and D/W (VR14, VR15), Volume (VR16).
Result: echoes from short slapback to long, decaying repeats — with the delay time under
voltage control.
Keyboard sensors
Capacitive Touch Interface (Keyboard) — uses two MPR121 capacitive touch sensor boards to provide 22 touch-sensitive keys.
Stage 1: MPR121 Capacitive Touch Sensors (S1 & S2). Each board has 12 electrode channels; Archean uses 11 on each. Board pins: 3.3V (pin 3), IRQ (pin 4) — interrupt output, active low; SCL (pin 5), SDA (pin 6) — I2C; ADD (pin 7) — address selection; GND (pin 8). Address configuration: S1 — the ADD jumper on the board is left in place (ADD to ground) → address 0x5A; S2 — the jumper is cut and ADD is connected to 3.3V → address 0x5B. Each electrode measures tiny capacitance changes when a finger approaches; on-chip baseline tracking and noise filtering give reliable touch and release detection. The electrodes connect through headers SV1 and SV2 to the keys on the front panel.
Stage 2: I2C Bus Connection. Shared SCL and SDA lines connect both MPR121 boards and the distance sensor to Teensy pins 16 (SCL1) and 17 (SDA1) — the Wire1 bus. The Teensy is the I2C master and reads both boards using their addresses (0x5A and 0x5B).
Stage 3: Interrupt (Q3, Q4, R37, R38, R39). The IRQ output of each board goes through R37 / R38 (10kΩ) to the base of Q3 / Q4. The transistors (collectors to +3.3V) share one emitter resistor R39 (4.7kΩ) to ground and combine both IRQ outputs into one MPR121_IRQ line to Teensy pin 4.
Distance sensor. Header S3 connects the VL53L0X module: power, GND, SCL, SDA (the same Wire1 bus), the interrupt output (to Teensy pin 2) and XSHUT.
Key Features: 22 touch keys, dual MPR121 with addressing via the ADD pin, interrupt signal for efficient reading, I2C — a simple two-wire connection to the Teensy.
Result: a responsive, expressive and futuristic touch interface that gives the Archean synthesizer its distinctive playable character.
Stage 1: MPR121 Capacitive Touch Sensors (S1 & S2). Each board has 12 electrode channels; Archean uses 11 on each. Board pins: 3.3V (pin 3), IRQ (pin 4) — interrupt output, active low; SCL (pin 5), SDA (pin 6) — I2C; ADD (pin 7) — address selection; GND (pin 8). Address configuration: S1 — the ADD jumper on the board is left in place (ADD to ground) → address 0x5A; S2 — the jumper is cut and ADD is connected to 3.3V → address 0x5B. Each electrode measures tiny capacitance changes when a finger approaches; on-chip baseline tracking and noise filtering give reliable touch and release detection. The electrodes connect through headers SV1 and SV2 to the keys on the front panel.
Stage 2: I2C Bus Connection. Shared SCL and SDA lines connect both MPR121 boards and the distance sensor to Teensy pins 16 (SCL1) and 17 (SDA1) — the Wire1 bus. The Teensy is the I2C master and reads both boards using their addresses (0x5A and 0x5B).
Stage 3: Interrupt (Q3, Q4, R37, R38, R39). The IRQ output of each board goes through R37 / R38 (10kΩ) to the base of Q3 / Q4. The transistors (collectors to +3.3V) share one emitter resistor R39 (4.7kΩ) to ground and combine both IRQ outputs into one MPR121_IRQ line to Teensy pin 4.
Distance sensor. Header S3 connects the VL53L0X module: power, GND, SCL, SDA (the same Wire1 bus), the interrupt output (to Teensy pin 2) and XSHUT.
Key Features: 22 touch keys, dual MPR121 with addressing via the ADD pin, interrupt signal for efficient reading, I2C — a simple two-wire connection to the Teensy.
Result: a responsive, expressive and futuristic touch interface that gives the Archean synthesizer its distinctive playable character.
Noise Generator
White Noise Generator — classic avalanche breakdown noise source using a single transistor, amplified for synthesizer use. Purpose: a broadband random signal for percussion (hiss, snares), wind/sea effects, or as a modulation source.
Stage 1: Avalanche Noise Source (Q2 — NPN transistor). Configuration: reverse-biased base-emitter junction; the base is grounded and the collector is not connected. R45 (100kΩ) — current-limiting resistor from +12V to the emitter. The avalanche effect generates broadband (white) noise at the emitter — a small noise voltage (a few millivolts) with a flat frequency spectrum. Note: noise amplitude varies between individual transistors; some are "noisier" than others.
Stage 2: High-Gain Amplifier (IC10A — TL072P). Configuration: non-inverting amplifier. C22 (470nF) — AC coupling capacitor from the emitter to the op-amp input; with R46 (100kΩ) to ground it passes frequencies above about 3.4 Hz. R48 (270kΩ) — feedback resistor. R47 (330Ω) — to ground. Gain = 1 + R48 / R47 = 1 + 270k / 330 ≈ 820. The TL072 provides a high-impedance input that does not load the noise source.
Stage 3: Output (C23, J10). C23 (47nF) — DC-blocking output capacitor: only the noise signal reaches the jack. J10 — output jack.
Touch pad. The meteorite pad on the front panel connects through header SV3 and R49 (330Ω) to the amplifier input. When you touch it, the electrical pickup of your body and the resistance of your skin mix into the input, and the noise changes its level and texture.
Key Features: transistor avalanche — true analog white noise; high gain stage (≈820×) — amplifies millivolt noise to a usable level; touch pad — expressive control of the noise.
Result: essential raw material for percussive hits, atmospheric textures, and unpredictable modulation in the Archean synthesizer.
Stage 1: Avalanche Noise Source (Q2 — NPN transistor). Configuration: reverse-biased base-emitter junction; the base is grounded and the collector is not connected. R45 (100kΩ) — current-limiting resistor from +12V to the emitter. The avalanche effect generates broadband (white) noise at the emitter — a small noise voltage (a few millivolts) with a flat frequency spectrum. Note: noise amplitude varies between individual transistors; some are "noisier" than others.
Stage 2: High-Gain Amplifier (IC10A — TL072P). Configuration: non-inverting amplifier. C22 (470nF) — AC coupling capacitor from the emitter to the op-amp input; with R46 (100kΩ) to ground it passes frequencies above about 3.4 Hz. R48 (270kΩ) — feedback resistor. R47 (330Ω) — to ground. Gain = 1 + R48 / R47 = 1 + 270k / 330 ≈ 820. The TL072 provides a high-impedance input that does not load the noise source.
Stage 3: Output (C23, J10). C23 (47nF) — DC-blocking output capacitor: only the noise signal reaches the jack. J10 — output jack.
Touch pad. The meteorite pad on the front panel connects through header SV3 and R49 (330Ω) to the amplifier input. When you touch it, the electrical pickup of your body and the resistance of your skin mix into the input, and the noise changes its level and texture.
Key Features: transistor avalanche — true analog white noise; high gain stage (≈820×) — amplifies millivolt noise to a usable level; touch pad — expressive control of the noise.
Result: essential raw material for percussive hits, atmospheric textures, and unpredictable modulation in the Archean synthesizer.